Counting scales are generally known in the art, and are typically used to determine the number of parts in batches of identical pieces. They are useful in many manufacturing, packaging and other commercial and industrial applications. Such scales can be particularly useful for determining the number of parts in large quantities of relatively small parts such as screws or washers, for example.
Counting scales basically operate by measuring the weight of a quantity of parts, and then computing the number of parts by dividing this measured weight by the weight of each individual part. Generally, the weight of each part, or piece weight, can be determined by a counting scale in a number of ways. For example, the piece weight can be calculated by a sample method, whereby a known quantity of parts are placed on the scale, and the piece weight is calculated by dividing the measured weight of the known quantity by the number of parts placed on the scale. This sample method can be a bulk sample method, where all the sample parts are placed on the scale at the same time, and the piece weight is computed when the scale becomes stable. Alternatively, a dribble sample method may be used, where the sample parts are counted onto the scale (i.e. it is not necessary to place them all on the scale at the same time) then a key on the scale is pressed to initiate the piece weight calculation. In either sample method, if the sample size is too small to meet a minimum desired accuracy, the counting scale will typically prompt an operator to add additional pieces. The piece weight can also be obtained by keying in a known piece weight, or by obtaining the piece weight from a database in which it has been previously stored.
One problem associated with many prior art high resolution counting scales is that they are often not particularly easy to operate. Many have rather cryptic operating systems, and their user interface is often difficult to master and understand. Also, many prior art high resolution counting scales are limited in such areas as expandability or connectivity-to other devices.
A need therefore exists for a counting scale which is easy to operate, and which takes advantage of many of the recent advances in user interfaces. Further, a need exists for a counting scale which is expandable and which is capable of communicating with a number of external devices, to create a "system" approach to counting scales.
Many counting scales seen in the prior art typically use an analog-type weighing device, such as a strain gauge, to measure weights. However, a number of drawbacks exist with these analog-type devices.
First, such analog-type devices are limited in accuracy and precision. Especially when dealing with very small parts, it is necessary to have great accuracy and precision in order to reliably compute the number of parts in a given sample. Strain gauge devices typically are limited to at best an accuracy of one division in six thousand. Also, the higher accuracy strain gauge devices can often take a number of seconds to settle and come to a stable weight reading. It is preferable in many manufacturing environments to obtain reliable counts as quickly as possible so that many batches of parts can be counted.
In addition, strain gauge type devices often require several minutes (as much as 30 minutes) upon initial power-up in order to give reliable and repeatable readings. It is preferred that a counting scale be usable as soon as possible after power-up so an operator does not have to wait long to use the scale.
Stain gauge devices are also sensitive to environmental conditions such as temperature, humidity and age, which may affect modulus, hysteresis, or anelastic material properties of the devices. Such sensitivity to the environment reduces the accuracy of whatever scales such devices are used in and requires counting scales using such devices to be recalibrated from time to time.
Therefore, there exists a need for a counting scale which is capable of accurately and precisely computing the number of pieces in a sample of unknown quantity. Further, a need exists for a counting scale which can count a number of very small parts quickly and accurately, and which has reduced sensitivity to environmental conditions.
It has been found that digital-type force transducers or load cells provide a number of advantages over analog-type strain gauge devices. Often these digital-type weigh devices provide more accuracy and precision, less sensitivity to environmental conditions, and quicker response times.
In the area of force measurement, a number of such load measuring devices and cells are known in the art. For example, Gallo, U.S. Pat. No. 4,043,190, discloses a meter for measuring mass or force wherein the sensed displacement acts indirectly on the tension of the two transversely vibrating electrically excited strings. Sette et al, U.S. Pat. No. 4,170,270, disclose an apparatus for preventing the overload of a load cell used to measure deflection. Blawert et al, U.S. Pat. No. 4,237,988, similarly disclose an overload protection device for precision, scales. Paros, U.S. Pat. No. 4,384,495, discloses a mounting structure for double bar resonators to ensure symmetrical loading of the resonator responsive to external forces. Also, Paros, U.S. Pat. No. 4,751,849 discloses various mounting structures for use with force sensitive resonators.
Further, Streater et al, U.S. Pat. No. 3,712,395, disclose a weight sensing cell which includes two differentially loaded vibrating members. Suzuki et al, U.S. Pat. No. 4,196,784, disclose a weighing scale having an interior load cell. Great Britain Patent No. 1,322,871 discloses a force measuring apparatus having a pretension string which is excited to a state of transverse oscillation by an electronic circuit. Gallo, U.S. Pat. No. 4,300,648, also discloses a meter for sensing mass and force comprising two flat springs lying in a parallel plane. Pulvari, U.S. Pat. No. 3,274,828, discloses a force sensor based on piezoelectric oscillators.
Also, Reid et al, U.S. Pat. No. 3,366,191, disclose a weighing apparatus which relies on a bridge circuit. Norris, U.S. Pat. No. 3,479,536, discloses a piezoelectric force transducer which is a piezoelectric vibratory beam mounted to receive compressive and tensile forces along its length. Agar, U.S. Pat. No. 3,529,470, discloses a force transducer having a composite strut with two bars which are to be maintained in transverse vibration at a common resonance frequency by electrical feedback wherein the frequency of vibration indicates the force applied to the composite strut. Corbett, U.S. Pat. No. 3,541,849, discloses an oscillating crystal force transducer. Wirth et al, U.S. Pat. No. 3,621,713, disclose an instrument for measuring masses and forces which when stressed by a load shows variation in frequency.
Saner, U.S. Pat. No. 3,724,572, Van de Vaart et al, U.S. Pat. No. 3,853,497, Melcher et al, U.S. Pat. No. 3,885,427, and Paelian, U.S. Pat. No. 3,915,248, all disclose a weighing system which functions by force or weight being transmitted to frequency sensitive elements. Meier, U.S. Pat. No. 3,963,082, Wirth et al, U.S. Pat. No. 4,088,014, Jacobson, U.S. Pat. No. 4,143,727, Ebbinge, U.S. Pat. No. 4,179,004, all disclose force sensing load cell.
Finally, Eer Nisse, U.S. Pat. No. 4,215,570, discloses a miniature quartz resonator force transducer having the shape of a double ended tuning fork. Ueda et al, U.S. Pat. No. 4,299,122, disclose a force transducer based on a vibrator having a pair of plate-shaped vibrating pieces parallel with each other. Paros et al, U.S. Pat. No. 4,321,500, disclose a longitudinal isolation system. Eer Nisse et al, U.S. Pat. No. 4,372,173, disclose a resonator force transducer which includes a pair of elongate generally parallel bars coupled at their ends with a double ended tuning fork arrangement.
Recently, quartz double-ended tuning forks have been used as force sensors in environments where the tension resisted the movement of the loaded structure, or the tension was produced by strain within the loaded structure. Levered systems and parallel guiding structures have been used where the force applied to the force sensing crystal was a fraction of applied load. The force sensing crystal was generally small since the force required to cause adequate frequency change in the resonant double-ended quartz tuning fork did not need to be great.
However, the loaded structure had to be massive to resist effects of undesirable lateral deflection. The flexing portions of these structures which acted as parallel bending beams or bending fulcrums carried some load since the force sensing crystal and its bonded joints deflected when tension was applied to the crystal.
The prior art load cells were dependent on the stability of the loaded structure and the bonding joints, over temperature and time, for output stability. For example, Albert, U.S. Pat. No. 4,838,369 discloses a load cell intended to provide a linear relationship between the signal generated and the force sensed. Albert uses a specific crystal design attached by screws to the frame of the load cell which creates a frictional joint resulting in inadequate zero return and cell precision. Albert relies on a longitudinally rigid structure to resist interferences from varying load positions. The load cell of Albert is designed so that force expended on the load cell, when stressed, results in work or energy loss within the screw joints. In turn, this phenomenon also results in poor zero return and precision.
Without attention to material similarly, non-strain sensitive designs, and reduction or cancellation of creep and hysteresis, Albert cannot provide a load cell which truly negates material and temperature effects.
Generally, material aging in these apparatus often caused long term performance to suffer after calibration. Further, these apparatus were limited in resolution by the degree in which anelastic creep and strain hysteresis were compensated for in their design. The quartz crystal bonding joints would often compensate for creep and hysteresis caused by the loaded structure with their own counteracting creep and hysteresis. When the quartz crystals were bonded using adhesives such as epoxies, stresses were introduced in the glue joints and crystal because of differential expansion between the base and the quartz and epoxy shrinkage during curing.
Further, as these stresses relaxed over time, the characteristics of the bonded joint changed because of the nonlinear stress-strain curve of the adhesive. This caused the load cell to have excessive zero and span shift over time until the glue joint stresses had relaxed. Differential expansion between the quartz and the structural material would cause the force sensor to have an output due to the temperature as well as applied load.
As a result, a need exists for a load cell which can compensate for changes in modulus of elasticity, anelastic creep, and strain hysteresis occurring in the elements of the cell due to stresses created by the environment of application.
Greater resistance to environmental conditions, and a resulting greater accuracy and precision, can be obtained by digitally processing the frequency outputs of a digital-type load cell. In the context of the present invention, a load cell or other digital-type force transducer, coupled with an associated driving and digital processing controller, comprises a "load cell assembly". The performance of such a unit may be enhanced by a suitable controller which can reject many environmental effects, and/or result in greater precision and accuracy.
In the area of frequency-to-digital conversion, for example, Check et al., U.S. Pat. No. 4,239,088, discloses a frequency-to-period converter which uses a frequency signal to gate a latch, which stores the output of a high-frequency counter. This enables the period of the load cell output to be calculated within one or two oscillations of the output. However, the counter shown in Check et al. must be reset after each period measurement. This reset may be time-limiting and may be unsuitable for very high frequencies. Also, by measuring the period of one or two output oscillations, the precision that may be obtained is also limited, and transient responses may widely vary until the output signal has stabilized.
Further performance improvements can be provided by linearization and temperature correction algorithms, which attempt to reject common-mode errors due to temperature and other effects. For example, Paros et al., U.S. Pat. No. 4,751,849, discloses a linearization routine for scaling the outputs of two sensing means in order to provide more accurate rejection of common-mode effects. Also, Eer Nisse et al., U.S. Pat. No. 4,535,638, discloses a routine for computing force and temperature using a single crystal oscillated at two separate frequencies.
Therefore, a need exists for a force transducer unit having a high resistance to environmental effects. Further, there exists a need for a controller for use in such a force transducer unit which provides further resistance to environmental effects beyond the mechanical characteristics exhibited by the load cell.